Deep eutectic solvents for the synthesis of covalent organic frameworks: from boundaries to possibilities

Covalent organic frameworks (COFs) are a class of crystalline porous materials assembled from discrete molecular building blocks, offering exceptional tunability and functionality that underpin their broad range of applications. However, conventional solvothermal routes for COF synthesis often rely on toxic organic solvents, high energy input, cumbersome degassing procedures and long reaction durations, posing significant environmental and scalability concerns. Deep eutectic solvents (DESs), composed of hydrogen-bond donor-acceptor pairs, have recently emerged as sustainable media for chemical synthesis. Due to their biodegradability, customizability, high thermal stability and unique solvation environment, DESs hold considerable potential to enhance the practical viability of COFs. Compared to conventional solvents used for COF synthesis, DESs can facilitate dynamic covalent reactions and eliminate the addition of catalysts, lowering reaction temperatures while simultaneously eliminating the need for degassing. Furthermore, the potential recyclability of DESs additionally reduces waste generation and enhances the economic viability of COF synthesis. In this review, DES-mediated COF syntheses of imine, β-ketoenamine, azine, and hydrazone linked two-dimensional (2D) and three-dimensional (3D) COFs are critically examined. The review further discusses the properties of the employed DESs, the structural features, characterization outcomes, advantages and limitations of the resulting materials, together with the underlying reaction mechanisms and product properties. Moreover, existing challenges are discussed, and future perspectives are proposed regarding the use of DESs to enable rapid, sustainable, scalable, and designable COF fabrication beyond conventional synthetic boundaries.